The attachment of metal ions to proteins, peptides and other, smaller molecules is a fast expanding technology, which has numerous proven and potential applications in research, in industry and, particularly, in medicine.
In recent years, much of the impetus behind the development of this technology has been the ability to link metal ions to antibodies, especially monoclonal antibodies. Such metal labelled antibodies have found a widespread use, especially in medicine, where they have been employed, for example, to target the metal ions to a specific tissue type, both in vitro and in vivo. Thus, metal labelled antibodies have applications in locating specific tissue types (e.g. employing computer-aided tomographic techniques where the metal ion is in some way detectable) and in the treatment of cell disorders (e.g. treating mammalian tumours where the metal ion is a cytotoxic radionuclide).
Conventionally, attachment of the metal ion to a protein such as an antibody has been achieved by complexation by an acyclic chelate such as a substituted diethylenetriaminepentaacetic acid [Gansow O. A. et al, Inorg. Chem., (1986), 25, 2772] or ethylenediaminetetraacetic acid [Meares, C. F. et al, Acc. Chem. Res., (1984), 17, 202] covalently linked to the antibody. Such acyclic complexes however tend to be unstable in vivo either as a result of acid-catalysed decomplexation or competitive chelate binding by Ca.sup.2+ or Zn.sup.2+ in serum, or as a result of competition from transferrin [Moerlein, S. M. et al, Int. J. Nuc. Med. Biol., (1981) 8, 277]. The lack of stability can result in uncomplexed metal atoms in the body which have a cytotoxic effect on healthy tissue (e.g. bone marrow) or which markedly reduce the signal-to-noise ratio of an imaging technique.
A possible alternative to the use of acyclic chelates in the labelling of antibodies is the use of macrocyclic ligands, which has previously been suggested in broad terms [Gansow ). A. et al, Am. Chem. Soc. Symp. Ser., (1984), 241, 215; UK patent Specification Publication No. 2122641; and Moi M. K. et al, Anal. Biochem., (1985), 148, 249-253]. More recently, tetra-azamacrocycles have been described which are capable of binding metals, and which can be conjugated to antibodies (International Patent Application Nos. WO 87/05030 and WO89/01476; and Moi M. K. et al J. Am. Chem. Soc., (1988), 110, 6266).
We have now found a new class of functionalised tetra-aza macrocycles, members of which are able to form kinetically inert complexes with metal ions. The macrocycles of the invention are particularly useful for attachment to proteins, especially antibodies, to provide conjugate compounds capable of binding metals, with good association rates, to give complexes which are advantageously stable in vivo and which posses an advantageous biodistribution profile.